This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-47769 filed on Feb. 23, 2001.
1. Field of the Invention
The present invention relates to a piezoelectric actuator drive circuit and a fuel injection system using the piezoelectric actuator drive circuit.
2. Related Art
A piezoelectric actuator functions based on the piezoelectric effect of a piezoelectric material such as PZT. A piezo-stack, which is a capacitive element, expands or contracts by being charged or discharged, thereby exerting force on a piston, etc. to render a linear motion. The piezoelectric actuator is used in a fuel injector of a fuel injection system for internal combustion engines.
However, if a conduction abnormality such as cable disconnection or contact failure to a connection terminal occurs, the piezo-stack cannot be charged or discharged and there results a malfunction of the piezoelectric actuator. As a result, fuel is not injected when fuel is to be injected. On the other hand, if the cable is disconnected after charging, the piezo-stack cannot be discharged and there results a continuous fuel injection even after the predetermined injection period. Some of the fuel injection system is provided with a fail-safe function that functions mechanically to stop the fuel injection after elapse of a certain time so that a serious failure can be avoided.
JP-A-1-202177 teaches to detect a voltage between both ends of a current detection resistor provided in a current supply path connected to the piezo-stack during the charging operation. If the detected voltage value is lower than a predetermined threshold value, it is determined that normal charging current does not flow due to conduction failure of the cable. The charging to the piezo-stack or a set of the piezo-stacks having a common cable is disabled. However, because the abnormality is detected based on the charge/discharge current, the abnormality detection timing is limited to the start or end of the injection period.
As a result, although the complete disconnection can be detected definitely, jittering in which disconnection and reconnection due to irregular contact to the connection terminal cannot always be detected. Moreover, in the case of jittering, which is different from the complete disconnection in which the piezo-stack already loses charge/discharge capability, it often happens that the fuel injection cannot be stopped due to disabled discharge even though the mechanical fail-safe function is operated.
It is an object of the present invention to provide a piezoelectric actuator drive circuit and a fuel injection system that is capable of accurately detecting a cable disconnection including jittering.
According to the present invention, a piezoelectric actuator drive circuit has a charge/discharge circuit section for charging and discharging a piezo-stack mounted on a piezoelectric actuator, a pair of positive and negative connection terminals connected to a current supply cable that extends to the piezo-stack, and an abnormality detection circuit section for detecting the conduction abnormality of the current supply cable. The abnormality detection circuit section includes a voltage detection circuit for detecting the voltage at the non-ground side connection terminal out of the connection terminals, and a comparison circuit for comparing the detected voltage with the reference voltage and for supplying a comparison signal.
The voltage at the non-ground side connection terminal falls down below the reference voltage and falls to 0V again when the piezo-stack starts to be discharged in response to a discharge command of the piezo-stack. If the current supply cable happens to be disconnected while the piezo-stack is in the charge holding state, the voltage at the connection terminal, that is, the detected voltage, falls down below the reference voltage even though the piezo-stack holds the charged state. The output of the comparison circuit is inverted resultantly. Thereby, the conduction abnormality of the current supply cable is detected regardless of charging or discharging. Because the detected voltage changes up and down across the reference voltage when jittering occurs, the jittering is detected easily.